In a traditional fluorescent tube lamp, the body of the lamp is a straight tube with a length of 15 to 60 times the diameter of the tube. Fluorescent tubes are low-pressure mercury discharge lamps in which the inner surface of the tube is coated with fluorescent material. The fluorescent tube lamp consists of an air-tight glass tube containing a small amount of mercury, an inert gas, a fluorescent coating, such as phosphor, and electrodes. At each end of the fluorescent tube, there is a lid with two symmetrically positioned contact pins, to which the electrodes are connected. Power supply to the fluorescent tube is provided via these contact pins. When the lamp is in operation, the temperature of the electrodes must be sufficiently high, so that electrons release from them. A fluorescent lamp does not ignite at a normal operation voltage without preheating. It is typical of fluorescent tubes that their cathodes are heated with separate preheat circuits or arrangements.
Retrofitting a fluorescent tube with a LED tube is nowadays becoming common. Retrofitting means replacing a fluorescent tube by a LED tube without altering the fluorescent tube lighting fixture except for removing or replacing the starter (if a passive-magnetic ballast is used) by a similar shaped device with a different function (this is optional).
When retrofitting a fluorescent tube with a LED tube a problem related to the risk of an electric shock during the mounting step of the LED tube occurs. According to the safety regulations in the field of electricity, fluorescent tube lighting fixtures are to be constructed in such a way that, when a fluorescent tube is replaced, it is not possible for a user to touch any parts at the mains voltage even if the fluorescent tube lighting fixture were carrying voltage. This requirement is met even if the fluorescent tube were replaced in such a way that only one end of the tube were in contact with the contacts of a tube holder of the fluorescent tube lighting fixture and the person replacing the tube can touch the other end of the tube. This requirement is met because no current goes through the gas-filled fluorescent tube before the gas in the tube is ionized with a starting pulse. In other words, the gas in the fluorescent tube serves as an insulator in itself. The electric structure of the lighting fixture, in turn, is such that generation of a starting pulse requires that both ends of the tube be connected to the contacts of the tube holders. This way the fluorescent tube prevents the risk of an electric shock during replacement.
With LED tubes, this electric safety requirement is not necessarily met. Inside LED tubes, there is a printed circuit or a corresponding structure, on which LEDs and electronic current supply components that they require are mounted. The purpose of these components is to convert the alternating voltage of the mains into direct voltage and to regulate the direct current required by the LEDs. In practice, current flows through these components once voltage is applied to them, in other words, the LED tube is in a conductive state without a separate starting pulse. Therefore, in a practical situation, when the LED tube is being mounted on a fluorescent tube lighting fixture, the contact pins at one end of the LED tube may hit the contacts of the tube holder of the fluorescent tube lighting fixture, and the other end of the tube may remain outside the fluorescent tube lighting fixture, so that the person mounting or replacing the tube may touch it, whereby he is prone to the risk of an electric shock.
US2011/0260614 discloses a LED tube for retrofitting in a fluorescent tube lighting fixture. The LED tube comprises a safety unit arranged to prevent a voltage from transferring through the tube lamp from its one end to the other until a voltage supplied from a corresponding tube holder of the fluorescent tube lighting fixture to the pair of contact pins has been separately detected at each end of the LED tube. Inside the LED tube, there is at least one optical line that is arranged to transfer a control or measurement signal associated with the safety unit from one end of the LED tube to the other without capacitive leakage currents. Although US2011/0260614 presents a solution to the above mentioned problem of the risk of an electric shook when retrofitting an LED tube in a fluorescent tube lighting fixture, the solution according to US2011/0260614 is complex and expensive due to the use of the optical line arranged to transfer a control or measurement signal associated with the safety unit from one end of the LED tube to the other.